Construction of an opposed-piston engine is well understood. In FIG. 1, the engine 8 illustrates an example of a two-stroke cycle, opposed-piston engine. The engine 8 includes one or more cylinders such as the cylinder 10. The cylinder 10 is constituted of a liner (sometimes called a “sleeve”) retained in a cylinder tunnel formed in a cylinder block. The liner includes a bore 12 and longitudinally displaced intake and exhaust ports 14 and 16, machined or formed in the liner near respective ends thereof. Each of the intake and exhaust ports includes one or more circumferential arrays of openings in which adjacent openings are separated by a solid portion of the cylinder wall (also called a “bridge”). In some descriptions, each opening is referred to as a “port”; however, the construction of a circumferential array of such “ports” is no different than the port constructions in FIG. 1.
One or more injection nozzles 17 are secured in threaded holes that open through the sidewall of the liner, between the intake and exhaust ports. Two pistons 20, 22 are disposed in the bore 12 of the cylinder liner with their end surfaces 20e, 22e in opposition to each other. For convenience, the piston 20 is referred to as the “intake” piston because of its proximity to, and control of, the intake port 14. Similarly, the piston 22 is referred to as the “exhaust” piston because of its proximity to, and control of, the exhaust port 16. The engine includes two rotatable crankshafts 30 and 32 that are disposed in a generally parallel relationship and positioned outside of respective intake and exhaust ends of the cylinder. The intake piston 20 is coupled to the crankshaft 30 (referred to as the “intake crankshaft”), which is disposed along an intake end of the engine 8 where cylinder intake ports are positioned; and, the exhaust piston 22 is coupled to the crankshaft 32 (referred to as the “exhaust crankshaft”), which is disposed along an exhaust end of the engine 8 where cylinder exhaust ports are positioned. In uniflow-scavenged, opposed-piston engines with a two or more cylinders, all exhaust pistons are coupled to the exhaust crankshaft and all intake pistons to the intake crankshaft.
Operation of an opposed-piston engine with one or more cylinders is well understood. Using the engine 8 as an example, each of the pistons 20, 22 reciprocates in the bore 12 between a bottom center (BC) position near a respective end of the liner 10 where the piston is at its outermost position with respect to the liner, and a top center (TC) position where the piston is at its innermost position with respect to the liner. At BC, the piston's end surface is positioned between a respective end of the cylinder, and its associated port, which opens the port for the passage of gas. As the piston moves away from BC, toward TC, the port is closed. During a compression stroke each piston moves into the bore 12, away from BC, toward its TC position. As the pistons approach their TC positions, air is compressed in a combustion chamber formed between the end surfaces of the pistons. Fuel is injected into the combustion chamber. In response to the pressure and temperature of the compressed air, the fuel ignites and combustion follows, driving the pistons apart in a power stroke. During a power stroke, the opposed pistons move away from their respective TC positions. While moving from TC, the pistons keep their associated ports closed until they approach their respective BC positions. In some instances, the pistons may move in phase so that the intake and exhaust ports 14, 16 open and close in unison. However, one piston may lead the other in phase, in which case the intake and exhaust ports have different opening and closing times.
One reason for introducing a phase difference in piston movements is to drive the process of uniflow scavenging in which pressurized charge air entering a cylinder through the intake port pushes the products of combustion (exhaust gas) out of the cylinder through the exhaust port. The replacement of exhaust gas by charge air in the cylinder is “scavenging.” The scavenging process is uniflow because gas movement through the cylinder is in one direction: intake-to-exhaust. In order to optimize the uniflow scavenging process, the movement of the exhaust piston 22 is advanced with respect to the movement of the intake piston 20. In this respect, the exhaust piston is said to “lead” the intake piston. Such phasing causes the exhaust port 16 to begin to open before the intake port 14 opens and to begin closing before the intake port. Thus, exhaust gas flows out of the cylinder before inflow of pressurized charge air begins (this interval is referred to as “blow down”), and pressurized charge air continues to flow into the cylinder after the outflow of exhaust gas ceases. Between these events, both ports are open (this is when scavenging occurs). Scavenging ends when the exhaust port 16 closes. Now, having no exit, the charge air that continues to flow into the cylinder 10 between time of closure of the exhaust port 16 and the time of closure of the intake port 14 is caught in the cylinder 10, and is retained therein when the intake port 14 closes. This retained portion of charge air retained in the cylinder by the last port closure is referred to as “trapped air”, and it is this trapped air that is compressed during the compression stroke.
Movement of the pistons in response to combustion is coupled to the crankshafts 30 and 32, which causes the crankshafts to rotate. The rotational position of a crankshaft with respect to a piston coupled to it is called the crank angle (CA). The crank angle is given as the angle from the position of the crankshaft to the centerline of the bore in which the piston moves; CA=0° when the piston is at TC. Presuming that the opposed-piston engine 8 is constructed for uniflow scavenging, a piston phase difference is established as per FIG. 2 by advancing the rotational position of the exhaust crankshaft 32 relative to the intake crankshaft 30 by some fixed amount, which is typically expressed as a “phase offset” in degrees of crankshaft rotation. This causes the exhaust piston 22 to lead the intake piston 20 by a corresponding amount throughout the operational cycle. During engine operation, the phase offset is maintained as the crankshafts rotate, and the crankshafts are said to be “phased.” More broadly, the term “phased crankshafts” refers to the two crankshafts of an opposed-piston constructed as per FIG. 1, in which the rotational movement of one crankshaft leads the rotational movement of the other crankshaft by a fixed number of degrees throughout the cycle of engine operation.
In FIG. 1, the pistons 20 and 22 are connected to the crankshafts 30 and 32 by respective coupling mechanisms 40 including journal bearings 42. In some aspects of two-stroke cycle engine operation, due to the nature of the cycle, a load reversal on a journal bearing may never occur during the normal speed and load range operation of the engine; or, the duration of a load reversal might be relatively short. In these circumstances, it is difficult to replenish the bearings with oil. Furthermore, given limited angular oscillation of the bearing, oil introduced between the bearing surfaces does not completely fill the bearing. Eventually the bearing begins to operate in a boundary layer lubrication mode (also called “boundary lubrication mode”), which leads to excess friction, wear, and then bearing failure. Related U.S. patent application Ser. No. 13/776,656 describes and illustrates a solution to the problem of non-reversing compressive loads that includes a rocking wristpin bearing (also called a “biaxial bearing”), which is incorporated into the engine 8 of FIG. 1. In this regard, each coupling mechanism 40 of the engine 8 may be constructed in a manner described in the '656 patent application and illustrated in FIG. 3. Referring to FIGS. 1 and 3, a coupling mechanism 40 supports a piston 20 or 22 by means of a rocking journal bearing 42 including a bearing sleeve 46 having a bearing surface 47 and a wristpin 48. The sleeve 46 is fixed to internal structure of the piston by conventional means. The wristpin 48 is retained on the small end 49 of a connecting rod 50 by threaded fasteners 51 for rocking oscillation on the bearing surface of the sleeve. The large end 53 of the connecting rod 50 is secured to an associated crankpin 54 of a respective one of the crankshafts 30, 32 by conventional fasteners (not shown). This structure is preferred, but is not intended to be limiting or to exclude other structures in which the wristpin is fixed and the sleeve is retained on the connecting rod for rocking oscillation on the wristpin. In either case, relative rocking oscillation occurs between the wristpin 48 and sleeve 46.
As seen in FIG. 4, the wristpin 48 is a cylindrical piece that comprises a plurality of axially-spaced, eccentrically-disposed journal segments. A first journal segment J1 comprises an annular bearing surface formed in an intermediate portion of the wristpin, between two journal segments J2. The two journal segments J2 comprise annular bearing surfaces formed at opposite ends of the wristpin, on respective sides of the journal segment J1. The journal segment J1 has a centerline A. The journal segments J2 share a centerline B that is offset from the centerline A. As per FIG. 5, the centerlines A and B are offset by equal distances from each other on a line 60 that is orthogonal to the longitudinal axis 62 of the connecting rod 50. As seen in FIG. 4, the sleeve 46 is a semi-cylindrically shaped piece with a segmented bearing surface that includes a plurality of axially-spaced, eccentrically-disposed surface segments. A first surface segment J1′ comprises an arcuately-shaped bearing surface formed in an intermediate portion of the wristpin, between two surface segments J2′. The two surface segments J2′ comprise arcuately-shaped bearing surfaces formed at opposite ends of the sleeve, on respective sides of the surface segment J1′. The surface segment J1′ has a centerline A′. The surface segments J2′ share a centerline B′ that is offset from the centerline A′ of surface segment J1′. As per FIG. 5, the centerlines A′ and B′ are offset by equal distances from each other on a line 60′ that is orthogonal to the longitudinal axis 62 of the connecting rod 50. The wristpin 48 is mounted to the small end 49 of the connecting rod 50 and the sleeve 46 is mounted to an internal structure of the piston, with bearing surface sets J1-J1′ and J2-J2′ in opposition.
In operation, as the piston to which they are mounted reciprocates between TC and BC positions, oscillating rocking motion between the wristpin 48 and the sleeve 46 causes the bearing surface sets J1-J1′ and J2-J2′ to alternately receive the compressive load. The segments receiving the load come together and the segments being unloaded separate, which enables a film of oil to enter space between the separating segment surfaces. A “load transfer point” occurs during oscillation of the bearing when the bearing surface sets are equally loaded and the direction of oscillation is causing the load to be increasingly transferred from one bearing surface set to another. During one full cycle of the two-stroke engine, this point is traversed twice, once when the piston moves from TC to BC, and again when the piston moves from BC to TC. As per FIG. 5, with 0° angular offset between the crankshafts, the load transfer points of the pistons occur at or near crankshaft positions of 0° (when the pistons are at their respective TC locations) and 180° (when the pistons are at their respective BC locations).
It has been recognized that positioning the load transfer point is important in the operation of traditional two-stroke engines with continuous compressive loads that have a peak cyclic intensity. For example, U.S. Pat. No. 3,762,389 discloses positioning a load transfer point to occur following the cycle peak load point (which occurs just after the piston TC position) so as to avoid minimization of the oil film between the bearing surfaces. However, with a single crankshaft and a single piston in each cylinder, each rocking journal interface is configured to the same load transfer point at the same time in each cycle.
What the '389 patent fails to consider is that setting all piston rocking journals to the same load transfer point in a two-stroke cycle, opposed-piston engine, with the exhaust crankshaft leading the intake crankshaft, will cause the same wristpin segments in the exhaust pistons to transition to an increasing highly loaded state further into the cycle and then diminish in loading as the pistons approach BC. When compared with the intake wristpin segments, this shift in loading of the exhaust wristpin segments will result in a lower minimum oil film thickness (MOFT) on the wristpin segment (J1 or J2) affected during the power stroke and higher MOFT on the segment that is loaded during the compression stoke, which is an undesired effect in a rocking journal lubrication scheme.